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Please if the card contains any important pictures, connect it to a computer and copy them to the computer if they are not backed up. Then put the card in the camera and format the card in the camera, then you can connect it back to the computer, put those pictures in there and then the card back into the camera if that is what you wanted to do.

Note that the camera might not show your pictures, unless they are named the same way as original pictures taken with the camera.

No. Water ruins cameras, water and damage due to being dropped is even worse. Now that you've let it sit and dry corrosion will have set into various electronic and mechanical components and there will be residual conductive salts left where the water evaporated.

Your camera is unrepairable. Buy another one and take better care of it, or buy a rugged tough camera designed to be "you proof" or given that it fell down a toilet, maybe that should be "poo proof"...

lets start with the basics, try hard reseting it and try again, if that doesnt work thentry upgrading your firmware (if possible on your camera) and if that doesnt work then it may be the lens, but dont forget to spend a few hours trying to find a option for it in the meny

Sigma lenses sometimes need re-chipping for full compatability with certain Nikon camera. They are usually quite good about doing this gratis.
Try the Sigma on another body at the camera shop or at home (preferably and older AF film camera). If it works then its likely just a chip compatibility issue.

It can be repaired, but usually water-damaged cameras are either worthless or VERY expensive to repair. I should expect that yours is destroyed, since Nikon's D200 is not designed to be waterproof, and has many electronic and steel parts which are very susceptible to being destroyed by the infiltration of water. A good camera shop would give you an estimate for no charge. If they tell you that it is beyond repair, you might take it to another shop for a second opinion. Is it possible for you to claim the damage on any insurance? You might check with your insurance agent just to be sure.